Compared with conventional liquid crystal panels, Active Matrix Organic Light Emitting Diode (AMOLED) display panels have many advantages such as high response speed, high contrast, wide visual angle, etc. The pixels of an AMOLED display panel are driven to emit light for displaying by related driving circuits on the array substrate. As shown in FIG. 1, a 2T1C pixel driving circuit in the prior art is shown. As can be seen from FIG. 1, the 2T1C pixel driving circuit in the prior art comprises two Thin Film Transistors (TFTs) and one capacitor, where the TFT M1 functions as a switch for controlling the connection between the data line and the gate of the TFT DTFT which is a driving TFT and can generate a driving current in the saturation state to drive the OLED to emit light. FIG. 2 is a timing diagram of the scan signal G(n) at the scan line and the gray-scale voltage signal Vd at the data line of the pixel driving circuit shown in FIG. 1. When the scan signal is at a low level, the TFT M1 is turned on and the gray-scale voltage Vd at the data line charges the capacitor C. When the scan signal is at a high level, the TFT M1 is turned off, the capacitor C is used to hold the gray-scale voltage. Since the positive voltage VDD of the power source is relatively high, the DTFT is at the saturation state. At this time, the driving current I of the OLED is:I=K(Vsg−|Vth|)2=K(VDD−Vd−|Vth|)2,where Vsg is the gate-source voltage of the DTFT, Vth is the threshold voltage of the DTFT, VDD is the positive voltage of the power source, Vd is the gray-scale voltage at the data line, K is a constant related to the transistor size and the carrier mobility which is determined by the TFT size and process.
However, in practical production process, even if the same process parameters are used, the threshold voltages of TFTs at different positions of the produced display panel may have large difference, such that the critical saturation voltages of the TFTs at different positions are also different, and thus the driving currents for OLEDs under the same gray-scale voltage are different. Therefore, the brightness at different positions of the display panel adopting this circuit would be different, resulting in poor brightness uniformity.